More than 1,300 Royal Mail employees have been caught stealing from the post in the last four years.
Postal workers are being prosecuted and sacked at the rate of more than one for every working day.
However, experts believe the true scale of the problem could be much greater.
Royal Mail is bombarded with complaints from customers who have not received deliveries they are expecting, from valuable mail order goods to birthday cards containing cash.
The organisation has repeatedly come under fire for failing to offer full compensation for items which are stolen, lost or damaged.
Following a Freedom of Information request, the Post Office revealed that 1,321 employees have been caught stealing since 2004.
Howard Webber, the chief executive of consumer group Postwatch, said: "It is shocking that on average one Royal Mail employee per day is being prosecuted or cautioned for theft.
"Postwatch has urged Royal Mail to publish these statistics regularly, so that we can all see whether the security of our mail is improving.
"It is in customers' interest that this is now done."
There are concerns that Royal Mail's reliance on temporary contract staff, particularly around Christmas, means the system is more vulnerable to theft.
The editor of the postal industry website, Hellmail, Steve Lawson, said: "Any cases of internal theft are enormously damaging and it may be that there is some correlation with cost-saving measures such as hiring of temporary staff or the trend towards replacing full-time staff with part-time.
"These statistics - including how much lost mail is the result of dishonest postal workers - should be more readily available if the 'slimming down' of our postal service is to have any credibility.
"Clearly delivery offices are stretched as Royal Mail tries to reduce operating costs.
"But there is a difference between running efficiently and allowing the reliability and integrity of mail delivery to fall apart at the seams."
Royal Mail pays out more than £1million a month in compensation to customers over failures in its services, including huge claims for stolen, lost and damaged items.
In the past, Postwatch has accused Royal Mail of 'cheating' customers by failing to pay proper compensation.
The organisation received complaints and compensation claims for some 651,582 lost items in the 2006/07 financial year, yet paid out on only 311,005.
The figures will make worrying reading for the Royal Mail's chief executive, Adam Crozier.
A spokesman from the company said: "Royal Mail takes the problem of theft extremely seriously and has a zero tolerance approach to dishonesty.
"This approach is shared by the overwhelming majorityof our people who are honest and hardworking and take great pride in getting mail safely to 27million addresses across the UK every working day.
"To put these figures in context, Royal Mail Group employs nearly 185,000 people in the UK."
He added: "The overwhelming majority of all letters posted arrive safely at the correct destination but we remain vigilant to any risk to the operation - including any threat from criminals outside of Royal Mail who target postmen and women to steal the mail they are carrying.
"When it comes to the very small fraction of mail that is stolen, the majority is taken by people outside of Royal Mail.
"It remains the case that the huge majority of our people are scrupulously honest and take huge care over the mail entrusted to them."
The number of staff prosecuted or cautioned each year is falling but it is difficult to know whether theft is becoming less common or whether Royal Mail does not have the resources to tackle it.
Those postmen and women who are caught stealing from the post have often carried out thousands of individual thefts.
For example, Nicholas Fryer was jailed for two years in 2004 after taking 130,000 letters and parcels over a five-year period and stashing them in his house and shed.
Prosecutors said the 31-year-old opened the packages he thought contained items of value.
The following year Lisa Harvey, 31, of Plymouth, was charged with stealing 100,000 pieces of mail.
The load was so huge it had to be taken away by a seven-ton truck.
